1. Field of the Invention
The present invention relates to biasing techniques in integrated circuit (IC) chips. In particular, the invention relates to biasing a floating node in a system that supports proximity communication.
2. Related Art
Advances in semiconductor technology presently make it possible to integrate large-scale systems, including hundreds of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip increases the speed at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays. Moreover, integrating large-scale systems onto a single semiconductor chip significantly reduces production costs, because fewer semiconductor chips are required to perform a given computational task.
However, these semiconductor chips still need to communicate with each other, and unfortunately these advances in semiconductor technology have not been matched by corresponding advances in inter-chip communication technology. Semiconductor chips are typically integrated onto a printed circuit board that contains multiple layers of signal lines for inter-chip communication. However, signal lines are typically 100 to 1000 times denser on a semiconductor chip than on a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This problem is creating a bottleneck that is expected to worsen as semiconductor integration densities continue to increase.
One solution to the above problem is to replace the direct conductive coupling with direct chip-to-chip capacitive coupling, referred to as “proximity communication.” Proximity communication is an I/O technology that allows two chips in face-to-face alignment to communicate without wires as has been explained by Drost et al. in “Proximity Communication,” IEEE Journal of Solid-State Circuits, vol. 39, no. 9, September 2004, pp. 1529-1535. In the most widely used implementation, corresponding arrays of electrode plates or pads are formed in the opposing surfaces of the two chips, which are then fixed together with a dielectric layer in between to form a large number of capacitively coupled communication links between the chips. One embodiment of a capacitively coupled communication system is illustrated in the circuit diagram of FIG. 1. A first integrated circuit chip, here called a transmit chip 10, includes a transmitter 12 as well as other integrated circuitry typical of a modern IC. A second integrated circuit chip, here called a receive chip 14, includes a receiver 16 as well as other integrated circuitry which needs to be coupled to the circuitry of the transmit chip 10. In this embodiment, the transmitter 12 and receivers 16 are differential, each having two inputs and two outputs for usually complementary versions of the same signal. Conductive transmit pads 18A, 18B are formed in the surface of the transmit chip 10 and are connected to the differential outputs of the transmitter 12 receiving an input signal VIN across its differential inputs. Similarly, conductive receive pads 20A, 20B are formed in the surface of the receive chip 14 in positions to be aligned with the transmit pads 18A, 18B of the transmit chip 10. The receive pads 20A, 20B are connected to the differential inputs of the receiver 16 outputting on its differential outputs an output signal VOUT, which should correspond to VIN.
Typically the pads 18A, 18B, 20A, 20B are covered with thin dielectric layers as part of their formation process. To achieve a proximity communication system, the transmit and receive chips 10, 14 are permanently or semi-permanently juxtaposed with the transmit pads 18A, 18B aligned with respective ones of the receive pads, 20B with a dielectric layer between them, thereby forming two capacitive coupling circuits 22, 24 between the two chips 10, 14 for the differential signal to be coupled between them.
However, the capacitive coupling circuits 22, 24 provide DC isolation between the two chips 10, 14 and the high input impedance of the differential amplifier of the receiver 16 provides very little conductive discharge to ground or other predetermined voltage. As a result, receiver nodes 26, 28 receiving the capacitively coupled signals are floating relative to the transmit chip 10 and not DC tied to the outputs of the transmitter 10. As a result, the receiver nodes 26, 28 may suffer DC wander due to leakage, and the input offset voltage of the receiver 16 subtracts from any signal coupled across the capacitors 22, 24.
In an alternative inductively coupled proximity communication system illustrated in the circuit diagram of FIG. 2, inductors 30, 34 are formed at the surfaces of the two chips 10, 14 and are covered with a thin dielectric layer. The transmit inductor 30 is connected across the differential outputs of the transmitter 12 and the receive inductor 32 is connected across the differential inputs of the receiver 16. When the two chips 10, 14 are juxtaposed with the two inductors 30, 32 in alignment, an inductive coupling circuit is formed between the two chips 10, 14. In the inductively coupled system, the receive nodes 26, 28 are also floating.
In either the capacitive or inductive proximity system, differential transmitters and receivers may be replaced by non-differential active elements having only a single input and single output. Thereby, one of the capacitive coupling circuits may be eliminated or the inductors have grounded ends. However, the differential implementations are preferred for the low signal levels inherent in proximity communication. Nonetheless, the single receive node remains floating unless counteracting measures are adopted.
Proximity communication involves integrating arrays of capacitive (or inductive) coupling circuits and associated transmitters and receivers onto semiconductor chips to facilitate inter-chip communication. The signal flow may be bi-directional so that transmitters and receivers formed in each chip and associated transmit and receive pads are formed in different surface areas of the chip. If a first chip is situated face-to-face with a second chip so that transmitter pads on the first chip are capacitively coupled via a coupling capacitor with receiver pads on the second chip, it becomes possible to transmit electrical signals directly from the first chip to the second chip without having to route the electrical signal through intervening signal lines within a printed circuit board.
A transmitter on one chip impresses an electrical signal on one of its pads, and a receiver on the other chip detects the signal coupled to the corresponding one of its pads. Although proximity communication promises much higher input/output (I/O) density and lower power, detecting signals over proximity communication is a challenge. Signals coupled onto the plates of the receiving chip can be very small, on the order of tens of millivolts, for example 50 mV. A typical 1σ deviation for the offset in the receivers is typically about 5 mV. In a system with one hundred such receivers, to achieve 99% reliability requires a 4σ range, which is 20 mV, so 40% of the signal is lost to offset. To achieve even these offsets, the input transistors of the receivers need to be very large because offset scales as
  1            length      ×      width      of the channel of the input transistor. The large input transistors require a large tail current in the amplifier to achieve sufficient amplifier gain and also reduce amount of signal VRX seen at the receiver input because the gate capacitance Cg acts as a capacitive voltage divider with the coupling capacitance Cc and the stray capacitance Cstray according to
      Δ    ⁢                  ⁢          V      RX        =      Δ    ⁢                  ⁢          V      TX        ⁢                            C          c                                      C            c                    +                      C            stray                    +                      C            g                              .      
With capacitively or inductively coupled proximity communication systems, the receive nodes 26, 28 should be actively biased at appropriate DC levels. However, prior offset cancellation schemes have added capacitance to the sensitive nodes in the proximity channel, thereby loading the sensitive receive amplifiers. They have also required dynamically refreshed nodes, thereby introducing pulsed operation and complex timing problems.
Previous schemes of offset biasing using additional capacitance have excessively loaded critical receiver nodes such as the internal or input nodes of the amplifier. The added load on the internal nodes reduces the performance of the amplifier and the added load on the input node increases the stray capacitance Cstray and thus reduces the amount of signal receive through the voltage division above.
Better schemes for mitigating the effect of the receiver offset should increase the sensitivity of the receivers and the performance of the proximity communication system over the conventional offset biasing techniques.